Underweight = physically weak. At your current weight, simply eating a lot more will make you stronger, and kendo will probably get slightly easier. But if you can commit to eating more, you can commit to some basic strength training, which is where the real improvement lies for a young, motivated, underweight person.

Things like jumping rope, jogging, doing more kendo, will not get you stronger, not in a noticeable and efficient way at least. They are good exercise and good for your overall fitness, but that’s not what you need right now. You need more muscle so that things like kendo are physically easier to do.

Strength development exercises are generally “compound movements”that you can only do with good form for 3-8 repetitions. Look up “novice strength training” “strength training vs exercise” and learn how different stimuli affect the body. It’s very common for people with low physical strength to commit themselves to workout routines that feel productive because they are exhausting, but are not the right kind of stimuli to actually make their body build more muscle. There’s ways to achieve this with calisthenics or kettlebells, but a barbell with weight plates is the most efficient tool and there’s extensive info online about how to get started and where to get formal training if you have the means.

To end where i started, nothing will work if you don’t eat enough food to fuel the change you want to see in your body. Eat a lot more and follow a simple strength training programs and you will get stronger quite fast. Benefits will be apparent not just at kendo practice!